1. Field of the Invention
The present invention relates to a belt device, and an image forming apparatus including the belt device.
2. Description of the Related Art
A conventional image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2004-318003 is a tandem type image forming apparatus applying an intermediate transfer method. In such method, color toner images formed respectively on four photosensitive elements are transferred onto an intermediate transfer belt in a superimposing manner. The superimposed toner images (a superimposed toner image) are transferred on a recording material. In the tandem type image forming apparatus, unless a surface moving speed of the intermediate transfer belt is constant, positions of respective color toner images are deviated from one another on the intermediate transfer belt. As a result, out of color registration is caused. Even if a driving source generating a rotational driving force transmitted to the intermediate transfer belt is rotated at a fixed angular speed, the surface moving speed of the intermediate transfer belt cannot be kept constant due to various causes. Therefore, feedback control to the driving source is conventionally performed so as to detect a rotational angular speed or a rotational angular displacement of an idle roller supporting the intermediate transfer belt and determine fluctuation of the surface moving speed of the intermediate transfer belt from the detection result to cancel the fluctuation. By conducting such control, fluctuation of the surface moving speed of the intermediate transfer belt can be suppressed, and out of color registration can be prevented accordingly. This arrangement can be similarly applied to a tandem type image forming apparatus applying a direct transfer method in which color toner images respectively formed on four photosensitive elements are transferred onto a recording material on a conveying belt.
To suppress the out of color registration, it is important to detect a rotational angular speed or a rotational angular displacement of a roller (a supporting rotational body) spanned with the intermediate transfer belt or the conveying belt with high accuracy. As a mechanism for the detection, one utilizing a rotary encoder is known. Specifically, a rotational disc (a rotational member) of a rotary encoder is attached to one end of a rotational shaft of a roller to be detected and it is rotated integrally with the rotational shaft of the roller. Plural slits (marks) moving on an orbit according to rotation of the rotational disc integrally with rotation of the rotational shaft are detected by a sensor (a mark detection unit). Based on a result of the detection, a rotational angular speed or a rotational angular displacement is detected.
However, as one of conventional detecting mechanisms utilizing a rotary encoder, a mechanism in which each of two ends of a rotational shaft of a roller is supported by only one supporting frame, and a rotary encoder is attached to one of the ends is known. In such a detecting mechanism, a roller vibrates due to driving of an intermediate transfer belt, a conveying belt, or the like, so that one end of the rotational shaft vibrates about a portion of the rotational shaft. Vibration propagates to the rotational disc and the sensor in the rotary encoder. Normally, timing of transmitted vibration reaching the rotational disc and reaching the sensor differs. Therefore, a relative positional relationship between the rotational disc and the sensor fluctuates. Since the fluctuation of the relative positional relationship appears as a detection error, this detecting mechanism has a problem that detection precision of the rotational angular speed or the rotational angular displacement of the roller is poor. In general, this problem is significant when the time interval of slit detection becomes shorter.
Another conventional detecting mechanism using a rotary encoder is disclosed in Japanese Patent Application Laid-open (JP-A) No. 2001-141736. This detecting mechanism also includes a rotational disc (a rotational member) rotated integrally with a rotational shaft of a roller spanned with an annular belt and a pattern detecting element (a mark detector) detecting a pattern (a mark) formed of plural slits in the rotational disc. The pattern detecting element is fixed on a supporting plate (a fixing member), and the supporting member is attached with a protective cover (a case). The rotational disc has a shaft portion extending in a normal direction to a plate face at a rotational central portion thereof, and the shaft portion is supported by both of the supporting plate and the protective cover. A fitting hole press-fitted with the rotational shaft of the roller is formed in the shaft portion. The rotary encoder is unitized so as to cover the pattern detecting element and the rotational disc using the supporting plate and the protective cover. When the rotary encoder is attached to the rotational shaft of the roller, the rotational shaft of the roller is press-fitted into the fitting hole of the shaft portion of the rotational disc. The supporting plate of the rotary encoder is fixed to a supporting frame that rotatably supports the rotational shaft by screws. One end of the rotational shaft attached with the rotary encoder is supported at three points by the supporting frame, and the supporting plate and the protective cover fixed to the supporting frame. Therefore, even if the roller vibrates due to driving of the intermediate transfer belt, the conveying belt, or the like, vibration of the rotational shaft supported by the supporting frame is suppressed compared to the case in which one end of the rotational shaft is supported at only one point by the supporting frame.
However, the detecting mechanism described in JP-A No. 2001-141736 has such a configuration that the supporting plate is fixed to the supporting frame at only one point on the supporting frame different from a portion thereon supporting the rotational shaft to facilitate assembling of the rotary encoder to the roller rotational shaft. Therefore, when the roller vibrates due to belt driving or the like, the supporting plate vibrates about a fixing portion of the supporting plate to the supporting frame due to the vibration. The vibration is propagated to the rotational disk and the sensor in the rotary encoder. Since a difference occurs between timings at which the vibration thus propagated reaches the rotational disc and the sensor, respectively, a relative positional relationship between the rotational disc and the sensor fluctuates. As a result, the fluctuation of the relative positional relationship appears as a detection error of the sensor like the conventional detecting mechanism where one end of the rotational shaft is supported at only one point of the supporting frame. Therefore, the rotational angular speed or the rotational angular displacement cannot be detected with high accuracy. In general, this problem is significant when the time interval of slit detection becomes shorter.
Such problem is not limited to a case of an image forming apparatus, and occurs in a general detection of a rotational angular speed or a rotational angular displacement of a supporting rotational body spanned with an annular belt.